1. Field of the Invention
The present invention is directed to the field of combustion process analyzers. More particularly, the present invention is directed to that portion of the above-noted field which is concerned with the analysis of the air/fuel ratio of the combustion mixture provided to for the combustion process. More particularly still, the present invention is directed to that portion of the above-noted field in which analysis of the chemistry of the exhaust gases produced by the combustion process, occurring for example in a furnace or an engine, results in a determination of the air/fuel ratio of the combustion mixture being provided to the combustion device. With greater particularity still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a sensor for accurately sensing the air/fuel ratio of a combustion mixture by changing an electrical characteristic in response to changes in the partial pressure of oxygen present in the exhaust gases resulting from the combustion process where the air/fuel ratio for the combustion mixture is to be maintained at a value in excess of the stoichiometric value.
2. Description of the Prior Art
The need for analyzing the operation of an internal combustion engine to improve or maintain the efficient operation thereof is well recognized. It has been suggested, at least as early as 1937, that the air-to-fuel ratio of a combustible mixture provided to an internal combustion engine may be determined by analyzing the composition of the exhaust gases produced from combustion of the mixture. An example of such a system appears in United States Pat. No. 2,077,538 -- "Exhaust Gas Analyzer For Automative Vehicles" issued in the name of L. S. Wait. According to the system disclosed in the Wait patent, a variable resistance element is situated in the exhaust stream of an internal combustion engine and arranged within an electrical bridge network, such as for example a Wheatstone bridge, so that the heat dissipative qualities of the variable resistance element will respond to changes in the chemistry of the exhaust gases to provide an indication of the air/fuel ratio of the combustion mixture being provided to the engine.
More recently, the efforts to reduce the pollution of our atmosphere have prompted implementation of stringent controls upon an automative engine exhaust emissions. In efforts to reduce the amount of pollutants injected into our atmosphere by mobile internal combustion engines, it has become apparent that accurate and self-adjusting control of the air/fuel ratio of the combustion mixture provided to such engines is desirable. For example, copending commonly assigned patent application Ser. No. 375,993, now U.S. Pat. No. 3,868,846 filed in the names of T. Kushida et al. and titled "Circuit For Converting A Temperature Dependent Input Signal To A Temperature Independent Output Signal" discloses one system wherein a titania exhaust gas sensor is responsive to changes in the exhaust gas chemistry, caused by excursions of the air/fuel ratio of the combustion mixture being provided to the engine from a selected value at, or richer, in fuel than, the stoichiometric value, to automatically and reliably control a controller means to maintain the air/fuel ratio of the combustible mixture at the selected value. It is also known that zirconia-based sensors may be similarly used. Other materials for use as exhaust gas sensor elements responsive to departures of a combustion mixture from a fuel rich value to a fuel lean value, and vice versa, by passing through the stoichiometric value are known.
The exhaust gases produced by fuel-rich combustion form an atmosphere which may be viewed as a reducing atmosphere. Many materials are known to respond electrically to changes in the chemistry of a reducing atmosphere. Exhaust gas sensors responsive to departure of a combustion mixture from the stoichiometric value to a fuel-rich or fuel-lean condition to produce a large scale change in an electrical characteristic are also known. Some of the reducing gas sensing materials also demonstrate this change which may ideally represent a step-function change. An example of this phenomenon is discussed in copending, commonly assigned patent application Ser. No. 393,698 now U.S. Pat. No. 3,886,785 and titled "Gas Sensor And Method of Manufacture", filed in the names of H. L. Stadler et al. The value of the stoichiometric responsive materials resides in this large scale excursion of an electrical parameter which may be utilized to directly and rapidly sense departure of the air/fuel ratio of the combustion mixture from stoichiometry.
It has become apparent that operation of an internal combustion engine with a combustion mixture at the stoichiometric value results in a combustion of exhaust gas pollutants which combination is expensive and difficult to correct by means of exhaust gas reactors. For example, combinations of two or more such reactors are known to be required to effectively reduce the exhaust gas pollutants produced under such conditions to values which the Environmental Protection Agency of the United States Federal Government asserts to be environmentally safe in internal combustion engine exhaust. While operation of the engine under either fuel-rich or fuel-lean conditions can overcome, in part, this difficulty, the presently available exhaust gas sensors function best at stoichiometric or under fuel-rich conditions and the resulting excess consumption of fuel is most undesirable. Operation of the engine under fuel-lean conditions is therefore preferrable since the wasteful excess consumption can be avoided. The exhaust gases produced under such a situation constitute an oxidizing atmosphere and it is therefore a specific object of the present invention to provide a sensor capable of operation in an oxidizing environment to function as an air/fuel ratio sensor.
It is also known that operation of heat producing furnaces, such as for heating, electric power generation or for industrial uses, may be efficiently operated with an air/fuel mixture which is fuel-lean. In view of the large quantities of various fuels consumed by such furnaces, accurate control of the air/fuel ratio to avoid excess consumption of fuel can have far reaching impact on total energy consumption. It is therefore a specific object of the present invention to provide a sensor for use in conjunction the air/fuel ratio control mechanism designed to maintain fuel delivery to a combustion process at a value which is fuel-lean of stoichiometry. More specifically it is an object of the present invention to provide a sensor for responding to exhaust gas combustion by-products to indicate the amount or condition of leanness of fuel delivered to a combustion process, as compared with the stoichiometric fuel quantity, whereby fuel delivery may be modulated to establish a desired, lean, air/fuel ratio for the combustion mixture supplied to a combustion process.
The operation of an internal combustion engine in the "lean regime", (that is, with a combustion mixture having an air/fuel ratio with a fuel control less than the stoichiometric fuel quantity for a given quantity of air, for example of from about 15 to about 22 in a gasoline powered internal combustion engine) and particularly at high values of the air/fuel ratio results in automatic lessening of the major exhaust gas pollutants (CO, HC and NO.sub.x). Removal of residual amounts of any of the major pollutants is therefore relatively simple. This relative simplicity of operation is predicated upon maintenance of the air/fuel ratio at a selected and precisely controlled value. Heretofore, reliable operation of the internal combustion engine within the lean regime at any one selected air/fuel ratio has been made virtually impossible due to the lack of a suitable sensor which could respond to variations in the air/fuel ratio from its selected value within the lean regime. It is therefore a specific object of the present invention to provide a sensor which is operative for combustion mixture within the lean regime and which may be used to accurately and reliably monitor the air/fuel ratio of the combustion mixture provided to an internal combustion engine. It is a further object of the present invention to provide such a sensor, and an electrical system responsive to the sensor, to produce a control signal which may be used to maintain a desired, known exhaust gas chemistry. It is a still further object of the present invention to provide such a sensor which has an approximately linear change in the logarithm of resistance in response to changes in the exhaust gas chemistry, and in particular to changes in the partial pressure of oxygen within the exhaust gas. More particularly still, it is an object of the present invention to provide a sensor suitable for positioning within the exhaust system of an internal combustion engine and having a sensing element or member with an electrical characteristic which varies in a controllable and predictable fashion in the presence of changes in the exhaust gas chemistry produced by variations in the air/fuel ratio of a fuel-lean combustion mixture provided to the engine. It is also an object of the present invention to provide an electrical system, including the sensor of the present invention, for controlling an operational amplifier to produce an output signal which may be used to control or modulate the air/fuel ratio of a combustion mixture.